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When they first rolled out Google+, they automatically added everyone on your circles to your chat list, if the other party had also enabled this chat setting. But this became a privacy issue and it was soon dropped.
Now the only way is to get their email address and add them manually to your chat list.

I guess now the setting is back on Google+ where you can have people on your circles back. But I guess, if you need to have them forever, you just need to ask them for their email ids.
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thunderrorMar 8 '12 at 18:48